Cr—W—V bainitic/ferritic steel compositions are of interest for high-strength and high-toughness applications. Please see U.S. Pat. No. 5,292,384 issued on Mar. 8, 1994 to Ronald L. Klueh and Philip J. Maziasz, entitled “Cr—W—V bainitic/ferritic steel with improved strength and toughness and method of making”, the entire disclosure of which is incorporated herein by reference.
There is usually a trade off in strength and toughness for most engineering materials: improved toughness usually comes at the expense of strength. The new ferritic steels have a bainite microstructure, and bainitic steels are generally used in the normalized-and-tempered or quenched-and-tempered conditions. Normalizing involves a high-temperature austenitizing anneal above the AC3 temperature (the temperature where all ferrite transforms to austenite on heating) and an air cool, and quenching involves the austenitization anneal and a water quench; tempering involves a lower-temperature anneal—below the AC1 temperature (the temperature at which ferrite begins to transform to austenite on heating). Tempering at higher temperatures and/or longer times at a given temperature improves the toughness at the expense of strength.
The objective, therefore, is to develop steels with optimized strength and toughness. Ideally, such steels would develop a low ductile-brittle transition temperature (DBTT) and high upper-shelf energy (USE) with minimal tempering (i.e., tempering at a low temperature or for a short time), thus allowing for high-strength and toughness. An ideal bainitic steel composition is one that can be produced by normalizing (air cooling) or quenching in water or other cooling media and then could be used without tempering. Economic considerations have made such steels a goal of the steel industry.
Early work on Fe-2.25Cr-2.0W-0.25V-0.1C (2 1/4Cr-2WV) demonstrated that by a proper heat treatment of Fe—Cr—W—V—C steels, it was possible to produce two different bainitic microstructures, shown in FIGS. 1a and 1b, in the normalized-and-tempered condition. It was discovered that the normalized-and-tempered microstructures developed during tempering were from two different bainite microstructures that formed during normalization; they were: carbide-free acicular bainite and granular bainite. The large blocky carbide particles that precipitate in the granular bainite are probably responsible for the inferior toughness of this steel.
Carbide-free acicular bainite consists of thin sub-grains containing a high dislocation density with an acicular appearance, shown in FIG. 2a. Granular bainite has an equiaxed appearance with bainitic ferrite regions of high dislocation density and dark regions, shown in FIG. 2b. The dark regions have been determined to be martensite and retained austenite and have been called “M-A islands” (martensite-austenite islands). They form because during the formation of the bainitic ferrite, carbon is rejected into the untransformed austenite. The last of the high-carbon austenite regions are unable to transform to bainite during cooling. Therefore, parts of these high-carbon regions transform to martensite when the steel is cooled below the martensite start (Ms) temperature. The remainder is present as retained austenite.
Whether carbide-free acicular bainite or granular bainite form during the normalization heat treatment depends on the cooling rate from the austenitization temperature. The difference in microstructure can be explained using a continuous-cooling diagram, shown in FIG. 3 (see for example, L. J. Habraken and M. Economopoulos, Transformation and Hardenability in Steels, Climax-Molybdenum Company, Ann Arbor, Mich., 1967, p. 69, R. L. Klueh and A. M. Nasreldin, Met. Trans. 18A, 1987, p. 1279; R. L. Klueh, D. J. Alexander, and P. J. Maziasz, Met. Trans. 28A, 1997, p. 335). If the steel is cooled rapidly enough to pass through Zone I in FIG. 3, acicular bainite forms; if cooled more slowly through Zone II, granular bainite forms.
Mechanical properties studies of the different bainites indicated that the acicular bainite had superior strength and toughness compared to the granular bainite. As an alternative to an increased cooling rate to achieve the favorable properties, it was concluded the same effect could be obtained if the hardenability was increased. To increase hardenability, the chromium and tungsten compositions were increased, and acicular bainite could then be produced in a 3Cr-2WV and 3Cr-3WV steel, whereas granular bainite was always produced for similar heat treatment conditions in the 2¼Cr-2WV steel, as shown in FIG. 4.